Wireless communication devices such as radiotelephones use antennas to transmit and receive radio frequency signals. Various types of antennas available for wireless communication devices include dipole antennas, helical antennas, and slot antennas. Slot antennas can be implemented with a gap in a metal surface. Simple resonant slot antenna geometries include a half wavelength (.lambda./2) slot antenna 110 as shown in prior art FIG. 1 and a quarter wavelength (.lambda./4) slot antenna 210 as shown in prior art FIG. 2. For a .lambda./2 slot antenna 110, the length 140 of the slot 120 is a half wavelength of the frequency of interest and both ends of the slot 120 are closed, while for a .lambda./4 slot antenna 210, the length 240 of the slot 220 is a quarter wavelength of the frequency of interest and only one end of the slot 220 is closed while the other end is open. The metal surface of the slot antenna is a ground plane 130, 230 that surrounds each slot 120, 220, and the antenna is driven differentially from positive and negative ports located near a closed end of the slot as shown.
To create a slot antenna that radiates in, for example, the 850 MHz frequency range, a .lambda./2 slot antenna 110 would have a slot length 140 of approximately 18 cm while a .lambda./4 slot antenna 210 would have a slot length 240 of approximately 9 cm. A 9 cm .lambda./4 slot antenna, unfortunately, is physically large for most hand-held radiotelephone applications. Thus, inductive loading has been developed, which slightly shortens the physical length of a slot antenna while maintaining the electrical length.
FIG. 3 shows a prior art quarter wavelength slot antenna 310 shortened using inductive loading. Slot antenna 310 includes a conductive ground plane 330 and is driven differentially from points near the closed end of the slot 320 as shown. The slot 320 has an area 350 where the width of the slot is larger. The configuration of area 350 can be generally rectangular as shown, or it can have other shapes such as circular. The width 370 and the length 360 of the area 350 create an increased impedance along length 360 of the slot. Depending on the length 360, width 370, and shape of the area 350, a five to ten percent reduction in slot length 340 can be achieved while maintaining radiation in the desired frequency band. Further reductions in length cannot be achieved due to physical limitations of the inductive loading technique. In other words, no part of the slot 320 can get wider than the width of the conductive surface that creates the ground plane 330. Also, the narrow section of ground plane that would be along the length 360 between two adjacent slot antennas with inductive loading may be difficult to fabricate.
FIG. 4 shows a prior art quarter wavelength slot antenna 410 shortened using a delay element with a high dielectric constant. A dielectric delay element 450 is inserted in series along a slot having a closed end and an open end. The delay element 450 can be fashioned in a variety of shapes and sizes to create the needed shortening effect. The ground plane of the slot antenna 410 is divided into three ground sections 430, 433, 436 by the delay element 450, and the slot is discontinuous and divided into two slot sections 421, 422 due to the delay element 450. The slot antenna is driven differentially from positive and negative nodes on ground section 430 near the closed end of the slot section 422 as shown.
The dielectric constant of the delay element 450 increases the overall phase delay of the slot antenna 410. Depending upon the length 460 of the delay element 450 and its dielectric constant, a ten to twenty percent reduction in slot length 440 can be achieved while still maintaining radiation in the desired frequency band. Impedance mismatches between the ground section 430, the delay element 450, and the ground sections 433, 436, however, cause undesired reflections that reduce the performance of the antenna.
The prior art inductive loading and delay element methods both furnish a limited decrease in slot length, however, not without some difficulties in manufacture. There is a need for a more dramatic decrease in the length of a slot antenna, and there is also a need for a shorter slot antenna that can be easily constructed to fit on a small wireless communication device such as a hand-held cellular radiotelephone.